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Bay Bottom History Lesson

"Anthropogenic disturbance" may sound like a weather forecast and "human perturbation" a sexual disorder but in a new study of San Francisco Bay's contaminant history they mean one simple thing: human activity. The study documents how human activities over time relate to concentrations of metals, PAHs, PCBs, DDT, and lead and radioactive isotopes in Bay sediments - drawing information from hundreds of cores drilled into the Bay bottom to trap sediment layers in profile. Many have long believed the Estuary too dynamic - too prone to sediment erosion, resuspension and movement - for this kind of study. But the U.S. Geological Survey's Michelle Hornberger says her results on metals show this simply isn't true. "In certain isolated pockets of the Bay we found a clear history," she says, "where metal concentrations taper down from the top of the core to the bottom." The relatively stable sediment pockets were in Richardson, Grizzly and San Pablo Bays.

Hornberger's research challenges another common assumption: that 1850s gold mining is to blame for the lion's share of Bay metal contamination. "The period of urbanization and industrialization had much more of an impact," says Hornberger, who found most of the metal enrichment above 70 centimeters (cm) in Richardson Bay and 120 cm in San Pablo Bay. These two depths are where telltale radioactive fallout from 1950s atomic weapons testing disappears. Hornberger also found higher metal concentrations in the North Bay than in Richardson Bay. This may stem from greater industrial discharges in the upper Estuary and more dilution by ocean sediments at Richardson.

Hornberger also discovered a linear relationship between metal enrichments she can trace to human activity (copper, lead and zinc) and two naturally accumulating metals (chromium and vanadium). Levels of the latter two metals don't vary with time, only with geography. "It may be possible to predict what the natural background level of a metal was in a particular part of the Bay based on the chromium and vanadium concentrations," she says. Knowing what metal levels were before people arrived is key to measuring the success of clean-up efforts, adds Hornberger. Her work, as part of the larger study due out soon, may offer the Bay's first record of contaminant history.

Contact: Michelle Hornberger (415)329-4467

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